MC Bowl Size Help Please

[avejoe]

Should the master cyl. on my K10 72 GMC with power brakes (front disk) have two bowls the same size or should the front bowl to the disks be bigger?

I continue to have all kinds of brake problems.

Thanks!

[4x4Poet]

Yep, disc brake caliper pistons take a larger volume of fluid to travel the distance within design parameters. Think about it. On 67-72 trucks, wheel cyl bores are about 1". 71-72 caliper piston bores are about twice that. Hence more fluid is used to move caliper pistons which requires more reserve fluid for safety stops after a leak starts.

I've run into most common brake problems on disc/drum truck setups. So have many others on this fine board. Throw it out there.

[avejoe]

OK...

Here's the latest.

I replaced the rear brakes yesterday. Emergency brake is adjusted.

Still can't bleed any fluid out the front disks. I used a c-clamp on the combination valve. Still no fluid.

The new master cyl. I put on last week had two bowls the same size and looked just like the one I took off. Thinking the new master might be bad, I got another master cyl. It has a single clamp wire (the old master had two) and it has two bowls of different sizes. I have not installed it yet because one of the two maters I have is wrong.

I can't assume the original master was the correct model as the previous owner could have mixed it up.

Now with the rear brakes new, I think the front brakes have never worked since I bought the truck.

Talked to a brake guy yesterday and he has never had a proportioning valve go bad so he does not think that is the problem.

I have not taken a front hose off yet because I can't get either caliper to bleed so I doubt I would get fluid there.



HELP

[Stocker]

Originally Posted by avejoe

Should the master cyl. on my K10 72 GMC with power brakes (front disk) have two bowls the same size or should the front bowl to the disks be bigger?

Like 4x4Poet says, the bowl to the calipers is larger because the pistons are larger than drum brakes' wheel cylinders. However, your wording above caught my eye. In my MC, the rear bowl is larger - and goes to the front discs.

Can't guess why you aren't getting fluid at the calipers, though. Maybe try removing a bleeder, or cracking open a brake line?

[avejoe]

So your K20 with front disks (power I assume), has the larger bowl in the back and it feeds the front? By rear bowl you mean the one closest to the firewall?

When I bleed my rear drums, the rear bowl drops. I have assumed this means the rear bowl feeds the rears. At this point anything is possible.

I can't imagine why my 72 GMC would have a different confuguration?????? Maybe the previous owner mixed everything up. Does your master cyl have one or two wires that hold the cap on?

Does the rear port on your master cyl feed to the rear (end opposite the button) port of your proportioning valve?

Thanks!

[Stocker]

Originally Posted by avejoe

So your K20 with front disks (power I assume), has the larger bowl in the back and it feeds the front? By rear bowl you mean the one closest to the firewall?

Yes.

When I bleed my rear drums, the rear bowl drops. I have assumed this means the rear bowl feeds the rears. At this point anything is possible.

When I compress the front calipers, fluid rises in the rear bowl.

I can't imagine why my 72 GMC would have a different confuguration?????? Maybe the previous owner mixed everything up. Does your master cyl have one or two wires that hold the cap on?

Two wires.

Does the rear port on your master cyl feed to the rear (end opposite the button) port of your proportioning valve?

No, the rear port of mc feeds the front port of prop. valve; and front of mc feeds rear of prop valve.

Hope this helps - you must be getting awfully frustrated with the entire project. Hang in there - you'll get it!

[Palf70Step]

Isaid the same thing about never having a proprtioning valve go bad either, but my 82 proved that to be wrong. Loosen the lines on your vavle and see if it bleeds through there, if not disconnect the input feed from the MC. If you got fluid in and none out, I'd say yuour vlave is bad. If it sat a while before you got it, there may have been moister in system causing it to rust. I also had both hose bad on the front discs also, which according a one mechanic I know could have also contributed to the proportioning valve going bad.

Some times I just hate messing with brakes. So many little things to aggrevate you

[avejoe]

I just got ahold of a 72 GMC service manual. On 1/2 tons the front bowl goes to the front of the prop valve and the front disks. On 3/4 tons the back bowl feed the front part of the valve. The manual shows the two bowls to be of equal size. I guess this way the same master would work on 1/2 and 3/4 tons?

I never thought brakes would be so difficult...

[4x4Poet]

Originally Posted by avejoe

...I have not taken a front hose off yet because I can't get either caliper to bleed so I doubt I would get fluid there....

That is an assumption. Bad calipers could prevent fluid from flowing through to the bleeders. You need to do the disconnect recommendations starting either at the master or the caliper. Prolly best to start with the master since that's the direction of fluid flow. Don't rule out the need for a bench bleed to start.